Heat transfer apparatus and method of manufacturing an integrated circuit and heat sink assembly

ABSTRACT

An improved apparatus and method for cooling integrated circuit assemblies uses a heat sink having a base and a displacement element having a size substantially similar to an area of heat concentration appropriately positioned on the integrated circuit. A compressive force placed upon the displacement element between the heat sink and the integrated circuit provides an optimum thermal resistance at an interface between the IC and the heat sink for efficient transfer of heat to the heat sink.

BACKGROUND

[0001] As microprocessors become faster and more powerful, they alsogenerate an increasing amount of heat. This heat must be dissipated tomaintain the optimum operating temperature of the component. Withoutproper heat dissipation, the microprocessor overheats and ceases tooperate. The microprocessor cooling effort is further complicated by thecommon practice of encasing the microprocessor. The practice of encasingthe microprocessor advantageously increases the durability of the partby protecting it from dust, dirt, and impact. The case conventionallyincludes a lid, also referred to as a “heat spreader”. The lid thatprotects the component typically has a larger surface area than themicroprocessor and also serves to distribute heat generated by themicroprocessor over the larger surface area of the lid. This heatdistribution is not even and there exists a localized area of heatconcentration on the lid just above the location of the microprocessor.The heat spreading function of the lid is insufficient to maintain themicroprocessor at an appropriate operation temperature. Accordingly,most microprocessors require an attached heat sink to draw the heat awayfrom the part and maintain the operating temperature.

[0002] There exist conventional heat sink designs that can properlydissipate the required amount of heat once the heat is transferred tothe heat sink from the heat source. If heat is not transferred fastenough, even a perfectly efficient heat sink cannot do the job and thepart will overheat. Traditionally, heat transfer from a heat source to aheat sink occurs by way of a mechanical communication. For example, athermally conductive area of the heat sink, which is typically a metal,is pressed against a thermally conductive area, also typically metal, ofthe heat source. Experience shows, however, that bare metal to metalcontact is not an efficient heat transfer mechanism. It has further beenfound that heat transfer can be improved by use of a thermal interfacematerial that is able to conform under pressure to fill small airpockets that exist between the heat source and the heat sink. Even thebest of thermal interface materials, however, do not transfer sufficientheat unless made extremely thin. Positioning a layer of thermalinterface material between a heat source and a heat sink requires thatthe thermal interface material be under a compressive force. In the caseof a microprocessor as the heat source, too much compressive force candamage the heat source itself or a printed circuit board to which themicroprocessor is attached. There remains a need, therefore, for anefficient heat sink that addresses the aforesaid challenges.

SUMMARY

[0003] An apparatus for removing heat from a heat source where the heatsource has an area of heat concentration comprises a heat sink having abase and a displacement element having a size substantially similar tothe area of heat concentration. A compressive force is placed upon thedisplacement element between the heat sink and the heat source.

[0004] An apparatus comprises a heat source with an area of heatconcentration, a heat sink, and a thermal interface material between theheat source and the heat sink. The apparatus further comprises a meansfor applying a compressive force on the thermal interface materialbetween the heat source and the heat sink and a means for concentratingthe compressive force on the area of heat concentration.

[0005] An apparatus comprises an integrated circuit generating heat andhaving a lid, the lid having a surface area larger than a surface areaof the integrated circuit resulting in an area of heat concentrationduring operation of the integrated circuit. The apparatus furthercomprises a heat sink and a displacement element having a surface areasized substantially similar to the area of heat concentration, and aspring clip. The spring clip places a compressive force on thedisplacement element between the heat sink and the lid.

[0006] A method for mounting a heat sink to a heat source comprises thesteps of providing a heat source and a heat sink, the heat source havingan area of heat concentration and determining an optimum size for adisplacement element as a function of the area of heat concentration.The method further comprises placing the optimally sized displacementelement between the heat source and the heat sink, and applyingcompression to the optimally sized displacement element between the heatsource and the heat sink.

[0007] A method of manufacturing an integrated circuit assemblycomprising the steps of providing a heat sink having a base, determininga size and position of an area of heat concentration on the integratedcircuit, and determining an optimum size for a displacement element as afunction of the area of heat concentration. The method further comprisesplacing the optimally sized displacement element between the integratedcircuit and the base, and applying compression to the optimally sizeddisplacement element between the integrated circuit and the base.

[0008] A method of manufacturing a printed circuit board assemblycomprising the steps of providing an integrated circuit mounted to aprinted circuit board, the integrated circuit requiring cooling duringoperation and having an area of heat concentration. The method furthercomprises providing a heat sink for the integrated circuit, determiningan optimum size for a displacement element as a function of the area ofheat concentration, and placing the optimally sized displacement elementbetween the integrated circuit and the heat sink. Compression is appliedto the optimally sized displacement element between the integratedcircuit and the heat sink.

[0009] An advantage of a heat dissipation apparatus according to theteachings of the present invention is efficient transfer and dissipationof heat generated by a heat source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a plan view of a conventional heat sink apparatus with acircled area representing the area shown in detail in FIG. 2 of thedrawings.

[0011]FIG. 2 is a cross sectional view of a conventional heat sinkapparatus as attached to an encased microprocessor as a heat source andshowing details of an interface between the heat sink and the heatsource.

[0012]FIG. 3 shows a typical uneven heat flux for a microprocessorillustrating the heat transfer and dissipation challenge addressed bythe teachings of the present invention.

[0013]FIG. 4 is a cross sectional view of a first embodiment of anapparatus according to the teachings of the present invention.

[0014]FIG. 5 is a cross sectional view of a second embodiment of anapparatus according to the teachings of the present invention.

[0015]FIG. 6 is a cross sectional view of a third embodiment of anapparatus according to the teachings of the present invention.

[0016]FIG. 7 is a graphical representation of the thermal resistance oftwo exemplary materials according to the teachings of the presentinvention.

[0017]FIG. 8 is example code for an analytical model according to theteachings of the present invention.

[0018]FIG. 9 is a graphical representation showing experimental data ascompared to data predicted by the analytical model.

[0019]FIG. 10 is a flow chart of a method for mounting a heat sink to aheat source according to the teachings of the present invention.

DETAILED DESCRIPTION

[0020] With reference to FIGS. 1 and 2, there is shown plan and crosssectional views, respectively, of a conventional heat sink apparatus 101in which thermal interface material is interposed at an interface 201between a heat sink base 202 and a heat source 203,204. The heat source203,204 may comprise a microprocessor 203 encased by a lid 204 or may beany other kind of heat source that requires efficient cooling. The heatsink may be of any conventional design that has a base 202 forattachment to the heat source 203, 204. The heat sink illustrated in thedrawings is an example of a particularly efficient heat sink and istaught in U.S. Pat. Nos. 5,785,116, 5,975,194, and 6,152,214 theteachings of which are hereby incorporated by reference. With referenceto FIG. 2, a thermally conductive gel is disposed in a cavity 205created by the lid 204 that encases the microprocessor 203 creating arelatively efficient transfer of heat from the microprocessor 203 to thelid 204. As shown in the illustration, the lid 204 presents an externalsurface area that is larger than an upper surface area of themicroprocessor 203. The heat sink base 202 covers most of the lid's 204surface area. A mechanism such as a spring clip, 102, places theinterface 201 with the thermal interface material in compression betweenthe base 202 and the lid 204. The compressive force causes the thermalinterface material 201 to thin and spread thereby reducing the thicknessof the thermal interface material 102.

[0021] With reference to FIG. 3 of the drawings, there is shown a topplan view of a microprocessor 203 encased in a lid 204 illustrating aheat flux at a moment in time just after the microprocessor is poweredand without any heat dissipation mechanism. FIG. 3 illustrates a typicalmicroprocessor having a top surface area measuring 9 mm by 11 mm encasedby the lid 204 having a top surface area measuring 31 mm by 31 mm. Aposition of the microprocessor 203 is shown in phantom line toillustrate the approximate size and position of the microprocessor 203relative to the lid 204. From the illustration, it is evident that thereis an area of heat concentration 301. The area of heat concentration 301is just above the position of the microprocessor 203 within the lid 204and corresponds is position and size to the size of the microprocessorheat source 203. In the absence of a heat dissipation mechanism and in asteady state condition, the entire lid 204 equalizes to the sametemperature, the microprocessor 203 overheats and the part fails. In thepresence of a heat dissipation mechanism, the thermal resistance of theinterface 201 permits transfer of the heat from the heat source 203/204through the thermal interface material to the heat base 202 for eventualdissipation of the generated heat into the air. Accordingly, the thermalinterface material in the interface 201 is heated in the process. Heatapplied to the thermal interface material may cause it to further flowand thin, advantageously further reducing the overall interface's 201thermal resistance.

[0022] The following relationship:$\frac{{Interface}\quad {thickness}}{\begin{matrix}{{Conductivity}\quad {of}\quad {the}\quad {interface}\quad {{material} \cdot}} \\{{Cross}\quad {sectional}\quad {area}\quad {of}\quad {the}\quad {interface}} \\{material}\end{matrix}} = \begin{matrix}{{Resistance}\quad {of}\quad {the}\quad {interface}} \\{{in}\quad {units}\quad {of}\quad {degree}\quad {C.\text{/}}{Watt}}\end{matrix}$

[0023] defines the expected thermal resistance of the interface.Accordingly, one of ordinary skill in the art appreciates that it isadvantageous to reduce the interface thickness to the smallest feasiblesize and maximize the cross sectional area of the interface material forminimum interface resistance and, therefore, maximum heat transferacross the interface 201 from the heat source 203,204 to the heat sink101. Compressive force on the thermal interface material 201 serves todecrease the thickness of the interface. The maximum compressive forcethat the heat source 203/204 is able to withstand is defined by thephysical properties of the encased part and is typically a finite value.This finite value, therefore, establishes the minimum achievableinterface resistance. An additional consideration is a bonding strengthof the interface 201 once compressive forces and heat is applied to thethermal interface material. The bonding strength of the heat sink to theheat source corresponds to a final area that contacts the base 202, thethermal interface material, and the heat source. The larger the finalarea of contact between base 202, thermal interface material, and theheat source, the stronger the bond between them. As the thermalinterface material is heated, it subsequently flows and defines thefinal area from which the bond strength results. It is advantageous forthe heat sink 1101 to be removable from the heat source 203/204 forreplacement as necessary. In many cases, however, applying the maximumcompressive force to the interface 201 causes the thermal interfacematerial to significantly thin and spread over a wide area. When thethermal interface material expands to cover an area as large as the lid204, the bonding strength of the interface often exceeds that of anattachment strength of the heat source 203/204 to a substrate 206, suchas a printed circuit board. Accordingly, the minimum practical thermalresistance is defined by the maximum compressive force that may beapplied without causing the bond strength of the interface 201 to exceedthat of the attachment strength of the heat source 203/204 to thesubstrate 206.

[0024] With reference to FIG. 4 of the drawings, there is shown a firstembodiment of an apparatus according to the teachings of the presentinvention in which the base 202 of the heat sink 101 further includes adisplacement element 401. The displacement element 401 in a preferredembodiment has a substantially similar surface area to the surface areaof the area of heat concentration 301. The displacement element 401 inthe first embodiment is unitary with the base 202 and comprises a smallstep, on the order of approximately 25-50 microns. The displacementelement 401 may be made by machining away a small height of basematerial leaving a circular central stepped section remaining. Thedisplacement element 401 is positioned over the area of heatconcentration 301 with thermal interface material disposed at theinterface 201 of the displacement element 401 and the heat source203/204. When the interface 201 is subject to the compressive force, thedisplacement element localizes the compressive force causing thepressure at the area of heat concentration 301 to be greater than thepressure applied for the same compressive force in the absence of thedisplacement element 401. The overall compressive force does not exceedthat which the lid 204 is able to withstand. The displacement element401, therefore, serves to localize the pressure on the lid 204 where itcan be most effective. The conformal thermal interface material in theinterface 201 responds to the pressure by thinning and spreading and thethickness of the interface 210 is reduced thereby advantageously alsoreducing the thermal resistance at the interface 201. When power isapplied to the microprocessor 203, the thermal interface material at theinterface 201 heats and further flows and thins, further reducing thethermal resistance of the interface 210. As the thermal interfacematerial flows, it flows past the displacement element 401, filling thespace between the lid 204 and the base 202. While the thermal resistanceoutside of the displacement element 401 is higher than the thermalresistance at the displacement element 401, it is sufficiently low topermit additional heat transfer from the heat source 203/204 to the base202, which provides an incremental advantage. Due to the fact that thearea subject to the highest pressure is smaller than the surface area ofthe entire base 202, the bonding strength of the interface is reducedrelative to prior art solutions. Accordingly, an apparatus according tothe teachings of the present invention improves the thermal resistanceof the interface 201 and therefore, the efficiency of the heat sink,while simultaneously addressing the issue of interface bonding strength.

[0025] With reference to FIG. 5 of the drawings, there is shown a secondembodiment of an apparatus according to the teachings of the presentinvention in which a supported thermal interface material 501 comprisesthe displacement element 401. An example of a supported interfacematerial 501 is Power Devices Co. Powerstrate Foil product and comprisesa foil based laminate with a conformal thermal interface material oneither side of the foil. There are various different versions of theproduct commercially available, each different version defining aparticular size. Advantageously, the supported material providessufficient displacement to concentrate the pressure from the compressiveforce at the area of heat concentration 301 according to the teachingsof the present invention in combination with the thermal interfacematerial itself. The pressure causes the interface material to conformto the irregularities in the base 202 and the lid 204 and to thin,thereby improving contact and reducing the thermal resistance of theinterface 201. Because the area of the thermal interface material issmaller relative to prior art solutions, this solution also reduces thebond strength of the interface 201 thereby improving the thermalresistance of the interface while also reducing the bond strength of theinterface.

[0026] With reference to FIG. 6 of the drawings, there is shown a thirdembodiment of an apparatus according to the teachings of the presentinvention in which an unsupported thermal interface material 601comprises the displacement element 401. An example of an unsupportedinterface material 601 is Berquist Co. HF225 product or Power DevicesCo. Powerfilm product and is similar or the same as the thermalinterface material used in prior art solutions. The difference in thisembodiment is that the unsupported thermal interface material 601 isplaced on or just inside the perimeter of the area of heat concentration301 instead of on the entire lid 204. While this solution iscounter-intuitive and does not follow the current teachings of the art,it is better than prior art solutions because the unsupported thermalinterface material 601 provides sufficient pressure increase at the areaof heat concentration 301 when compressive force is applied to theinterface 201 to thin out the interface thickness thereby reducing thethermal resistance. The reduction in interface thickness more thancompensates for the decreased cross sectional area and higher thermalresistance outside of the area of heat concentration 301. Additionally,the reduced surface area of thermal interface material also reduces thebond strength permitting removal of the heat sink from the lid 204 forreplacement of the microprocessor 203.

[0027] With reference to FIG. 7 of the drawings, there is shown a graphrepresenting the relationship between thermal resistance as shown on theY-axis 701 of the graph and an initial surface area of the thermalinterface material as shown on the X-axis 702 of the graph. For all datapoints on the graph, the mechanism that applies the compressive force tothe interface 201 is the spring clip 102 and is the same for allmeasured data points. An unsupported thermal interface material curve703 shows that an area of thermal interface material that does not coverthe area of heat concentration 301 shows a higher thermal resistancethan an area of thermal interface material that covers an areasubstantially equal to the area of heat concentration 301. This behavioris not unexpected because the equation shows that an increase in crosssectional area of the thermal interface material reduces the thermalresistance of the interface. It is interesting to note, however, that asthe initial area of thermal interface material increases, for theunsupported thermal interface material 601, the thermal resistance ofthe interface increases. It has been found that the reason for this risein thermal resistance is that for a given amount of compressive force,the larger initial surface area thermal interface material samplespresent more resistance to thinning at the interface 201. Because thethermal resistance of the interface is directly and proportionallyrelated to the thickness of the interface, the fact that the givencompressive force is not able to thin the interface is not compensatedby the larger cross sectional area of the thermal interface material.Additionally, the larger initial surface area of the thermal interfacematerial results in a higher interface bond strength, which does notpermit removal of the base 202 from the heat source 203, 204.Accordingly, it has been found that there is an optimum initial surfacearea size and position for maximum heat transfer.

[0028] A supported thermal interface material curve 704 shows that anarea of thermal interface material that does not cover the area of heatconcentration 301 shows a higher thermal resistance than an area ofthermal interface material that covers an area substantially equal tothe area of heat concentration 301. The difference is more pronouncedthan in the unsupported thermal interface material samples shown incurve 703, because the interface 201 is already quite thin and the crosssectional area of the thermal interface material is more of a factor. Asthe initial surface area of the thermal interface material increases forthe supported thermal interface material 501, there is very littlechange in thermal resistance. It has been found that this occurs becausethe supported thermal interface material 501 is already thin, thecompressive force does not further thin it out to any significantdegree. The larger initial surface area of the supported thermalinterface material, however, does increase the bond strength of theinterface 201 and it is advantageous to keep the initial cross sectionalarea of the thermal interface material to the minimum necessary toachieve the desired thermal resistance. Accordingly, there is an optimumsize and position of both thermal interface materials that followsimilar guidelines for slightly different reasons. It is expected thatother conformal and thermally conductive materials will behave similarlyand follow similar guidelines are presented in the present disclosurewhen used for optimum heat transfer.

[0029] Based upon the teachings herein, it is possible to develop ananalytical model of the behavior of the various embodiments ofdisplacement element and thermal interface material and thereby predictan optimum size of thermal interface material. By using no more than theoptimum amount of thermal interface material, the bond strength of thebase 202 to the heat source via the thermal interface material can beminimized without compromising heat transfer.

[0030] The analytical model may be implemented as a computer programthat accepts information including fixed value characteristics of theheat source and thermal interface material. The analytical model thenestablishes constraining equations for the thermal interface materialand then solves and presents the optimum size for the thermal interfacematerial of interest. With specific reference to FIG. 8 of the drawings,there is shown example code written for the software applicationentitled “Engineering Equations Solver (EES)” by f-Chart Softwarerunning on a Windows operating system for an analytical model accordingto the teachings of the present invention. The example code illustratesfixed values for an area of the lid (A_lid) 801, an initial thickness ofthe thermal interface material (ti) 802, a value for the compressiveforce applied (F) 803, and the conductivity constant of the thermalinterface material (k) 804 are defined. In the illustrative example, thevalues represent the surface area of the heat source 204 and thecompressive force presented by the clip 102 that attaches the heat sink101 to the heat source 204. The analytical model then defines a seriesof constraining equations that quantify the characteristics and physicalproperties acting upon the thermal interface material and include thefollowing:

[0031] An initial area of the thermal interface material (A_i) 805calculated from the length (l) 806 of the material. The presentanalytical model assumes a square piece of thermal interface material.

[0032] An initial volume of the thermal interface material (V) 807 iscalculated by determining a product of the initial thickness (ti) 802and the initial area of the thermal interface material (Ai) 805. A finalvolume is calculated by determining a product of a final thickness (t_m)808 and final area of the thermal interface material (A) 809. Becausethe initial volume and the final volume are the same, these twoequations define one of the constraining relationships of the analyticalmodel.

[0033] Pressure (P) 810 that is placed on the thermal interface materialis calculated as the force (F) 803 divided by the final area of thethermal interface material (Af_in2) 811 in inches squared.

[0034] The thickness of the thermal interface material (t) 812 may bedefined as a function of the compressive pressure (P) 810 applied to it.In one example the thickness of the thermal interface material (t) 810may be defined and converted into units of meters as:t_m=(−6.0989*ln(P)+55.2)*convert(micron,m).

[0035] In order to assure that the analytical model accounts for thesituation where the thermal interface material thins to the point wherethe area expands beyond the limits of the lid, an “Afinal” function 820may be defined that will return the value of the calculated area (A) orthe value of the surface area of the lid (A_lid), whichever is smaller.This check is somewhat of a verificaiton function because thermalinterface material that flows past the perimeter of the lid 204 is nolonger available as a heat transfer agent and must therefore be takenout of the equation. The verified value, i.e. the final calculated areaor the total area of the lid, is returned as the final surface area ofthe thermal interface material (Af) 813.

[0036] The analytical model further takes into account the presence of anon-uniform heat source 203/204 where there is an area of heatconcentration 301 that is a fraction of the total surface area of theheat source 203/204. The analytical model disclosed represents the areaof heat concentration as an eta factor 814 where the area of heatconcentration is equal to the surface area of the lid (A_lid) 801multiplied by the eta factor 814. The eta factor 814 as disclosedestimates the behavior of a non-uniform heat source by assuming a twopart heat variance; an area of heat concentration with an outer areawithout heat. One of ordinary skill in the art, however, can use thepresent teachings to formulate and effectively use other factors andassumptions to define the area of heat concentration 301 consistent withthe purposes and heat source behaviors at issue. When the effective areais calculated, another check is performed to assure that the final areaof the thermal interface material does not exceed the total area of thelid (A_lid) 801.

[0037] Although the analytical model disclosed is implemented in thesoftware application entitled Engineering Equation Solver (“EES”) byf-Chart and runs on a Windows operating system, other softwareapplications and calculated methods may also be used without departingfrom the teachings of the present invention. To use the analyticalmodel, the disclosed model is run using the “calc min/max” functionminimizing theta 815. Theta 815 is the thermal resistance of the thermalinterface material and is defined as the final thickness (t_m) 808divided by the product of the conductivity constant (k) 804 of thethermal interface material multiplied by the final surface area of thethermal interface material. The constraining equations in the analyticalmodel provide a value for the optimum initial surface area of thermalinterface material. The analytical model disclosed optimizes the thermalresistance by varying the initial area of the thermal interfacematerial. As one of ordinary skill in the art can appreciate, however,the teachings of the present invention can be used to minimize thermalresistance of the interface by varying other factors such as force (F)803 and used as appropriate. With specific reference to FIG. 9 of thedrawings, there is shown a graphical representation of the experimentalbehavior 901 of the thermal interface material (shown as data pointsalong the graph) and the predicted behavior 902 of the thermal interfacematerial (shown as a curve along the graph). In the graph, the x-axis903 represents an initial (i.e. uncompressed) length of one side of thethermal interface material and the y-axis 904 represents thermalresistance 815. The graph shows that the analytical model closelymatches the experimental behavior for initial lengths greater thanapproximately 0.325 square inches. In both cases, both the experimentaland analytical models show an optimum initial length. It has been foundthat the optimum initial length coincides with the final size of thermalinterface material that just covers the area of heat concentration 301.

[0038] With specific reference to FIG. 10 of the drawings, there isshown a flow chart according to the teachings of the present inventionincluding use for the analytical model presented herein. A first step1001 of the method is to provide the heat source 203,204 and a heat sink101. Thereafter, the method comprises establishing values andconstraining functions in an analytical model that represents behaviorof a thermal interface material under compressive pressure 1002. Usingthe analytical model, the method continues by determining an optimuminitial size for the thermal interface material by minimizing a thermalresistance of the thermal interface material at its final surface areaunder the defined constraints 1003. Using the resulting optimum initialsize for the thermal interface material, positioning the optimally sizedthermal interface material between a base 202 of the heat sink 101 andthe heat source 203/204 at step 1004. The last step comprisescompressing the optimally sized thermal interface material between thebase 202 of the heat sink 101 and the heat source 203/204 to achieve thepredicted minimum thermal resistance 1005. This analytical model may beused for the embodiment wherein the displacement element is the thermalinterface material. In this case, the optimum initial size of thermalinterface material may be placed appropriately and also act as thedisplacement element to concentrate the compressive force at the area ofheat concentration. This analytical model also may be used to determinean optimum size for the displacement element when it is a separateelement from the thermal interface material. In this case, an optimumsize for the displacement element corresponds to the value of the finalsurface area of the thermal interface material.

[0039] Embodiments described herein illustrate the invention by way ofexample. For example, materials different than those mentioned and heatdissipation mechanisms different than those pictured may be substitutedwhile still following the teachings of the present invention. Variationsof the claimed invention are within the capability of one of ordinaryskill in the art given benefit of the prior art and the presentdisclosure and are, therefore, within the scope of the appended claims.

1. An apparatus for removing heat from a heat source, the heat sourcehaving an area of heat concentration, the apparatus comprising: a heatsink having a base, a displacement element having a size substantiallysimilar to said area of heat concentration, and a compressive forceplaced upon said displacement element and between said heat sink andsaid heat source.
 2. An apparatus as recited in claim 1, saiddisplacement element further comprising a stepped base on said heatsink.
 3. An apparatus as recited in claim 1, wherein an unsupportedthermal interface material comprises said displacement element.
 4. Anapparatus as recited in claim 3 wherein said unsupported thermalinterface material comprises Berquist Co. HF225 product.
 5. An apparatusas recited in claim 1 wherein said heat source comprises an integratedcircuit.
 6. An apparatus as recited in claim 1 wherein said heat sourcefurther comprises a microprocessor having a lid with a surface arealarger than a surface area of said microprocessor, said lid interposedbetween said microprocessor and said displacement element.
 7. Anapparatus as recited in claim 1, wherein a spring clip between said heatsink and said heat source applies said compressive force upon saiddisplacement element.
 8. An apparatus comprising: a heat source havingan area of heat concentration, a heat sink, a thermal interface materialbetween said heat source and said heat sink, a means for applying acompressive force on said thermal interface material between said heatsource and said heat sink, and a means for concentrating saidcompressive force on said area of heat concentration.
 9. An apparatus asrecited in claim 8, said means for concentrating said compressive forcecomprising a stepped base on said heat sink.
 10. An apparatus as recitedin claim 9 wherein said thermal interface material covers said surfacearea of said heat source.
 11. An apparatus as recited in claim 8 whereina supported thermal interface material comprises the combination of saidmeans for concentrating said compressive force and said thermalinterface material.
 12. An apparatus as recited in claim 11 wherein saidsupported thermal interface material comprises Power Devices Co.Powerstrate Foil product.
 13. An apparatus as recited in claim 8,wherein an unsupported thermal interface material comprises thecombination of said means for concentrating said compressive force andsaid thermal interface material.
 14. An apparatus as recited in claim 13wherein said unsupported thermal interface material comprises BerquistCo. HF225 product.
 15. An apparatus as recited in claim 8 wherein saidheat source comprises an integrated circuit.
 16. An apparatus as recitedin claim 8 wherein said heat source further comprises a microprocessorhaving a lid with a surface area larger than a surface area of saidmicroprocessor, said lid interposed between said microprocessor and saiddisplacement element.
 17. An apparatus as recited in claim 8, wherein aspring clip between said heat sink and said heat source comprises saidmeans for applying said compressive force upon said thermal interfacematerial.
 18. An apparatus comprising: an integrated circuit generatingheat and having a lid, said lid having a surface area larger than asurface area of said integrated circuit resulting in an area of heatconcentration during operation of said integrated circuit, a heat sink,a displacement element having a surface area sized substantially similarto said area of heat concentration, and a spring clip placing acompressive force on said displacement element between said heat sinkand said lid.
 19. An apparatus as recited in claim 18, said displacementelement comprising a stepped base on said heat sink.
 20. An apparatus asrecited in claim 19 wherein a thermal interface material covers anentire surface area of said lid.
 21. An apparatus as recited in claim18, wherein a supported thermal interface material comprises saiddisplacement element.
 22. An apparatus as recited in claim 21 whereinsaid supported thermal interface material comprises Power Devices Co.Powerstrate Foil product.
 23. An apparatus as recited in claim 18,wherein an unsupported thermal interface material comprises saiddisplacement element.
 24. An apparatus as recited in claim 23 whereinsaid unsupported thermal interface material comprises Berquist Co. HF225product.
 25. An apparatus as recited in claim 18 wherein said integratedcircuit comprises a microprocessor.
 26. A method for mounting a heatsink to a heat source comprising the steps of: providing a heat sourceand a heat sink, said heat source having an area of heat concentration,determining an optimum size for a displacement element as a function ofsaid area of heat concentration, placing said optimally sizeddisplacement element between said heat source and said heat sink, andapplying compression to said optimally sized displacement elementbetween said heat source and said heat sink.
 27. A method for mounting aheat sink to a heat source as recited in claim 26 wherein saiddisplacement element comprises a thermal interface material.
 28. Amethod for mounting a heat sink to a heat source as recited in claim 27,said step of determining further comprising the steps of establishingvalues that represent fixed characteristics for said thermal interfacematerial during said step of compressing, establishing constrainingequations for an initial size of said thermal interface material, athickness of said thermal interface material as a function of saidcompression, and a thermal resistance of said thermal interfacematerial, wherein said step of determining said optimum size for saiddisplacement element comprises minimizing said thermal resistance valueof said thermal interface material.
 29. A method for mounting a heatsink to a heat source as recited in claim 28, said step of determiningan optimum size further comprising providing an eta factor for defininga characteristic for a non-uniform heat source.
 30. A method formounting a heat sink to a heat source as recited in claim 29 whereinsaid eta factor defines a subset of a surface area of said heat sourcethat represents an area of heat concentration.
 31. A method for mountinga heat sink to a heat source as recited in claim 26 wherein saiddisplacement element comprises a stepped base on said heat sink and saidstep of determining further comprises determining an optimum size for athermal interface material for placement on said displacement element.32. A method for mounting a heat sink to a heat source as recited inclaim 31, said step of determining further comprising the steps ofestablishing values that represent fixed characteristics for saidthermal interface material during said step of compressing, establishingconstraining equations for an initial size of said thermal interfacematerial, a thickness of said thermal interface material as a functionof said compression, and a thermal resistance of said thermal interfacematerial, wherein said step of determining said optimum size for saiddisplacement element comprises minimizing said thermal resistance valueof said thermal interface material.
 33. A method for mounting a heatsink to a heat source as recited in claim 31, said step of determiningan optimum size further comprising providing an eta factor for defininga characteristic for a non-uniform heat source.
 34. A method formounting a heat sink to a heat source as recited in claim 33 whereinsaid eta factor defines a subset of a surface area of said heat sourcethe represents said area of heat concentration.
 35. A method ofmanufacturing an integrated circuit assembly comprising the steps of:providing a heat sink having a base, determining a size and position ofan area of heat concentration on said integrated circuit, determining anoptimum size for a displacement element as a function of said area ofheat concentration, placing said optimally sized displacement elementbetween said integrated circuit and said base, and applying compressionto said optimally sized displacement element between said integratedcircuit and said base.
 36. A method of manufacturing an integratedcircuit assembly as recited in claim 35 wherein said displacementelement comprises a thermal interface material.
 37. A method ofmanufacturing an integrated circuit assembly as recited in claim 36,said step of determining an optimum size further comprising the steps ofestablishing values that represent fixed characteristics for behavior ofsaid thermal interface material in response to compression, establishingconstraining equations for an initial size of said thermal interfacematerial, a thickness of said thermal interface material as a functionof said compression, and a thermal resistance of said thermal interfacematerial, wherein said step of determining said optimum size for saiddisplacement element comprises minimizing said thermal resistance valueof said thermal interface material.
 38. A method of manufacturing anintegrated circuit as recited in claim 37 wherein said integratedcircuit is a microprocessor encased in a lid.
 39. A method ofmanufacturing an integrated circuit assembly as recited in claim 35,said step of determining an optimum size further comprising providing aneta factor for defining a characteristic for a non-uniform heat source.40. A method of manufacturing an integrated circuit assembly as recitedin claim 39 wherein said eta factor defines a subset of a surface areaof said heat source the represents an area of heat concentration.
 41. Amethod of manufacturing an integrated circuit assembly as recited inclaim 35 wherein said displacement element comprises a stepped base onsaid heat sink and said step of determining an optimum size furthercomprises determining an optimum size for a thermal interface materialfor placement on said displacement element.
 42. A method ofmanufacturing an integrated circuit assembly as recited in claim 41,said step of determining an optimum size further comprising the steps ofestablishing values that represent fixed characteristics for saidthermal interface material during said step of compressing, establishingconstraining equations for an initial size of said thermal interfacematerial, a thickness of said thermal interface material as a functionof said compression, and a thermal resistance of said thermal interfacematerial, wherein said step of determining said optimum size for saiddisplacement element comprises minimizing said thermal resistance valueof said thermal interface material.
 43. A method of manufacturing anintegrated circuit assembly as recited in claim 42, said step ofdetermining an optimum size further comprising providing an eta factorfor defining a characteristic for a non-uniform heat source.
 44. Amethod of manufacturing an integrated circuit assembly as recited inclaim 43 wherein said eta factor defines a subset of a surface area ofsaid heat source the represents an area of heat concentration.
 45. Amethod of manufacturing an integrated circuit assembly as recited inclaim 35 wherein said integrated circuit is a microprocessor.
 46. Amethod of manufacturing an integrated circuit assembly as recited inclaim 45 wherein said microprocessor is encased in a lid.
 47. A methodof manufacturing a printed circuit board assembly comprising the stepsof: providing an integrated circuit mounted to a printed circuit board,said integrated circuit requiring cooling during operation and having anarea of heat concentration, providing a heat sink for said integratedcircuit, determining an optimum size for a displacement element as afunction of said area of heat concentration, placing said optimallysized displacement element between said integrated circuit and said heatsink, and applying compression to said optimally sized displacementelement between said integrated circuit and said heat sink.
 48. A methodof manufacturing a printed circuit board assembly as recited in claim 47wherein said displacement element comprises a thermal interfacematerial.
 49. A method of manufacturing a printed circuit board assemblyas recited in claim 48, said step of determining an optimum size furthercomprising the steps of establishing values that represent fixedcharacteristics for behavior of said thermal interface material inresponse to compression, establishing constraining equations for aninitial size of said thermal interface material, a thickness of saidthermal interface material as a function of said compression, and athermal resistance of said thermal interface material, wherein said stepof determining said optimum size for said displacement element comprisesminimizing said thermal resistance value of said thermal interfacematerial.
 50. A method of manufacturing a printed circuit board assemblyas recited in claim 49 wherein said integrated circuit comprises amicroprocessor encased in a lid.
 51. A method of manufacturing anintegrated circuit assembly as recited in claim 49, said step ofdetermining an optimum size further comprising providing an eta factorfor defining a characteristic for a non-uniform heat source.
 52. Amethod of manufacturing an integrated circuit assembly as recited inclaim 51 wherein said eta factor defines a subset of a surface area ofsaid heat source the represents an area of heat concentration.
 53. Amethod of manufacturing an integrated circuit assembly as recited inclaim 47 wherein said displacement element comprises a stepped base onsaid heat sink and said step of determining an optimum size furthercomprises determining an optimum size for a thermal interface materialfor placement on said displacement element.
 54. A method ofmanufacturing an integrated circuit assembly as recited in claim 53 saidstep of determining an optimum size further comprising the steps ofestablishing values that represent fixed characteristics for saidthermal interface material during said step of compressing, establishingconstraining equations for an initial size of said thermal interfacematerial, a thickness of said thermal interface material as a functionof said compression, and a thermal resistance of said thermal interfacematerial, wherein said step of determining said optimum size for saiddisplacement element comprises minimizing said thermal resistance valueof said thermal interface material.
 55. A method of manufacturing anintegrated circuit assembly as recited in claim 54, said step ofdetermining an optimum size further comprising providing an eta factorfor defining a characteristic for a non-uniform heat source.
 56. Amethod of manufacturing an integrated circuit assembly as recited inclaim 55 wherein said eta factor defines a subset of a surface area ofsaid heat source the represents an area of heat concentration.
 57. Amethod of manufacturing an integrated circuit assembly as recited inclaim 47 wherein said integrated circuit is a microprocessor.